1. Field of the Invention
This invention relates to the field of bidirectional scanning in television receivers, video monitors and other raster scanned displays.
2. Statement of Related Art
Direct view cathode ray tube (CRT) display systems employ magnetic deflection and unidirectional scanning using a sawtooth to current with a fast flyback. With the introduction of high definition television (HDTV) and signal processing techniques improving picture quality, displays with higher horizontal scanning rates, for example up to 64 kHz, are desirable. A scanning rate of 64 kHz corresponds to 4f.sub.H, where f.sub.H is a conventional horizontal scanning rate, for example is 15.734 kHz. Due to the short flyback time, sawtooth line deflection results in high energy losses, for example, approximately 70 watts (W) at 64 kHz, strong radiation problems and increased loading of the line output transistor. These drawbacks can be overcome by using bidirectional deflection, where both forward and backward scans are used for video display. The general principle of bidirectional scanning is shown in FIG. 1. Lines of the raster are alternately scanned in the forward direction (left to right) and in the reverse direction (right to left). There is no flyback, as in a forward only scanned raster. Vertical deflection is constant during each line scan.
The main problem in bidirectional deflection systems is to achieve a good alignment of two adjacent lines to avoid the zip-fastener effect. A bidirectional raster with good alignment is shown in FIG. 2(a). A bidirectional raster with bad alignment, evidenced by the zip-fastener effect, is shown in FIG. 2(b).
Bidirectional scanning in accordance with prior art techniques utilizes a sinusoidal horizontal deflection waveform.